1. Field of the Invention
The present invention relates to a lubricating oil composition, and more particularly, to a lubricating oil composition which is excellent in sludge formation inhibiting effect, and hence is suitable especially for use as an engine oil.
2. Background Art
In a gasoline engine, sludge is formed due to the oxidative deterioration of an engine oil at high temperatures, and the reaction between the engine oil and unburnt fuel, blow-by gas (especially NO.sub.x), or the like. The sludge formed causes engine troubles such as blockage of an oil path or a valve, and an increase in viscosity of the engine oil. Therefore, the engine oil is required to be capable of inhibiting the sludge formation as much as possible so as not to cause the engine troubles. The engine oil has been used under very severe conditions with the trend toward the higher output of a gasoline engine and the smaller capacity of an oil pan for the engine, oil for the purpose of saving energy especially in recent years. Accordingly, a higher sludge formation inhibiting effect has been required of the engine oil.
The engine oil is generally manufactured by adding additives such as ashless dispersant, friction inhibitor, and metallic detergent to a lubricating base oil. In general, polybutenylsuccinimide has been used as the ashless dispersant in conventional engine oils. However, the sludge formation inhibiting effect exerted by polybutenylsuccinimide has still been unsatisfactory for establishing the technology to increase the life of the engine oil.